The present invention relates to the field of the protection of telecommunication equipment against electromagnetic interference of the fast transient type.
An item of equipment for which such protection is relevant is of the type which includes first and second connectors for connection to first and second lines respectively, and a multilayer printed circuit, one region of which supports signal processing components, each of the first and second lines including at least two electrical conductors, the printed circuit incorporating electrical links between the connectors and said components.
Typically, it may be a digital network equipment such as a digital network termination (NT) or else a repeater. A NT is an item of equipment which is placed in the vicinity of a digital subscriber site (for example on an apartment landing) and which provides for the signal transformations, as required by the digital transmission protocols and formats employed, between a network line connected to an interface of the NT referred to as the U interface and a subscriber line connected to another interface of the NT referred to as the S interface.
It is generally specified that an item of equipment such as a NT may be close (a few centimetres) to an electrically earthed element (for example the reinforcing bars of a reinforced-concrete wall on which would be installed a compartment intended for receiving the equipment). Under these conditions, the electromagnetic disturbances caused by shutdowns of electric motors, or of other inductive loads, situated near the equipment may be induced by coupling in the lines attached to the equipment. Such interferences may be of large amplitude with very short rise times (several hundred volts in a few nanoseconds). They flow away to earth, dying out relatively rapidly in the cables. However, it is advisable to prevent them from being able to reach the signal processing components of the equipment, else they would bring about transmission errors.
In general, such protection is achieved by providing for the electrical links between the connectors of the equipment and the signal processing components to be inductive, small inductance coils being mounted for this purpose on the printed circuit close to the connectors. However, the stray capacitances existing between the adjacent turns of a coil reduce its impedance with regard to the fastest disturbances, thus substantially limiting the effectiveness of the protection.
An object of the present invention is to propose simple and economical means for efficiently immunizing the equipment against intense and rapid interference.